Antifuse cells are popular in the field of semiconductors, where they are often used as an implementation of a one-time programmable cell. For instance, they can be used for purposes such as recording of secret codes, production numbers, etc. In particular, an antifuse cell is a structure that can be used to record a digital value, such as 0 or 1, by creating, or not, an electrical connection between two electrodes. More specifically, by applying a high voltage between the two electrodes, a layer of insulator is broken and connection between the electrodes is achieved. Antifuse cells are, therefore, typically write-once memories.
FIG. 5A illustrates an antifuse cell 5000A in accordance with the state of the art. Such cell is described, for instance, in non-patent literature “Comparison of embedded non-volatile memory technologies and their applications,” Linh Hong, Kilopass (retrieved from the from the World Wide Web at kilopass.com).
More specifically, the antifuse cell 5000A comprises a semiconductor substrate 5100 on which two transistors 5200, 5300 are realized: a program transistor 5200 comprising gate 5210 and gate oxide 5220, and a select transistor 5300 comprising gate 5310 and gate oxide 5320. The two transistors 5200 and 5300 are connected in series via a first connection region 5110. The other end of select transistor 5300 is connected to a second connection region 5120, which is then connected to a connection 5130.
The programming of the antifuse cell 5000A is carried out in the following manner. Contact 5130 is at a positive voltage and transistor 5300 is in an “on” state. When a high voltage is applied on the program transistor 5200, the oxide 5220 below the gate 5210 will break and a permanent electrical connection will be realized between the gate 5210 and the first connection region 5110. In this manner, if the high voltage is applied, a digital value of, for instance, 1, is recorded. Conversely, if the high voltage is not applied, a digital value of, for instance, 0, is recorded.
The reading of the antifuse cell 5000A is carried out by opening the select transistor 5300 with the application of the required voltage on its gate 5310. In this manner, the first connection region 5110 is connected to the second connection region 5120 and to the connection 5130. Therefore, by applying a voltage between the gate 5210 and the connection 5130, it is possible to detect the value stored in the antifuse cell 5000. In particular, with reference to the example above, if a current is flowing between the gate 5210 and the connection 5130, then a digital value of 1 is read. If no current is flowing, then a digital value of 0 is read.
This implementation requires the usage of two transistors next to each other, as well as the presence of several connection regions, which take up a considerable area on the semiconductor substrate 5100.
FIG. 5B illustrates an alternative antifuse cell 5000B in accordance with the state of the art.
Antifuse cell 5000B is advantageous over antifuse cell 5000A in that it does not require a first connection region 5110. More specifically, semiconductor substrate 5100B comprises only one connection region, namely second connection region 5120. This is achieved by realizing transistors 5200 and 5300 next to each other, such that they do not need a connection region in between.
However, such arrangement means that the high voltage used during the programming phase will be applied to both gates 5210 and 5310. This would result in the oxide below the select transistor 5300 to be damaged too. In order to solve this problem, the select transistor 5300 is replaced by select transistor 5300B, which is provided with a gate oxide 5320B thicker than the gate oxide 5220 of the program transistor 5200.
While this solution reduces the area by eliminating the need for first connection region 5110, it requires the usage of two different gate oxide thicknesses. This usually results in the problem that the select transistor 5300B, having the thicker gate oxide, cannot be realized with the minimum feature pitch, thereby becoming bigger than select transistor 5300, which increases again the area used by antifuse cell 5000. Additionally, the presence of two different gate oxides requires one additional mask as well as some manufacturing steps, increasing costs.